NPTL makes internal use of the first two real-time signals (signal numbers 32
and 33). One of these signals is used to support thread cancellation and POSIX
timers (see timer_create(2)); the other is used as part of a mechanism
that ensures all threads in a process always have the same UIDs and GIDs, as
required by POSIX. These signals cannot be used in applications.

To prevent accidental use of these signals in applications, which might
interfere with the operation of the NPTL implementation, various glibc library
functions and system call wrapper functions attempt to hide these signals from
applications, as follows:

At the Linux kernel level, credentials (user and group IDs) are a per-thread
attribute. However, POSIX requires that all of the POSIX threads in a process
have the same credentials. To accommodate this requirement, the NPTL
implementation wraps all of the system calls that change process credentials
with functions that, in addition to invoking the underlying system call,
arrange for all other threads in the process to also change their credentials.

The implementation of each of these system calls involves the use of a real-time
signal that is sent (using tgkill(2)) to each of the other threads that
must change its credentials. Before sending these signals, the thread that is
changing credentials saves the new credential(s) and records the system call
being employed in a global buffer. A signal handler in the receiving thread(s)
fetches this information and then uses the same system call to change its
credentials.

POSIX says that any thread in any process with access to the memory containing a
process-shared (PTHREAD_PROCESS_SHARED) mutex can operate on that
mutex. However, on 64-bit x86 systems, the mutex definition for x86-64 is
incompatible with the mutex definition for i386, meaning that 32-bit and
64-bit binaries can't share mutexes on x86-64 systems.

This page is part of release 4.15 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the latest
version of this page, can be found at
https://www.kernel.org/doc/man-pages/.